1. Technical Field
This disclosure relates generally to protective gear and, more particularly, to personal protective gear, such as helmets, including one or more dampers to protect against impacts.
2. Description of the Related Art
The performance of protective gear, such as, for example, protective headgear in the form of helmets, is especially important when the risk and nature of the injuries is more severe. Impacts to the head, for example, can lead to mild or traumatic brain injuries that can lead to long-term and cumulative impairments. Various helmet standards and assessments are known to qualify the level of a helmet's performance. A helmet's impact performance is typically assessed by the acceleration measured within a helmeted headform during an impact. Most standards consider only linear, direct impacts, not oblique impacts or other impacts causing rotational acceleration. Rotational acceleration is believed to be an important factor in many concussions and traumatic brain injuries. Moreover, many current standards evaluate only higher velocity impacts more relevant to skull fractures than milder concussions, which are of growing concern.
Most helmets and other personal protective equipment use crushable materials or structures to manage impact forces. Examples of crushable foam include expanded polystyrene (EPS), Expanded Polypropylene (EPP) or thermoplastic blown foam. Examples of crushable structures include those shown in U.S. Pat. Nos. 7,673,351 and 8,069,498, and U.S. Patent Application Publication No. 2010/0258988. These crushable foams and structures have several performance shortcomings. Primarily, they are generally rate insensitive and nonlinear in their response. They can only be “tuned” to a limited range of impact velocities, such as those usually necessary to pass certification standards, so they may not adequately protect in lower velocity impacts that may nevertheless result in concussions. They generally respond non-linearly during an impact. For example, there is often a delay following impact before such materials start significantly managing impact energy. Crushable materials and structures generally act like non-linear springs and most rebound too strongly after reaching peak displacement. This increases the duration of acceleration, which degrades or compromises a helmet's impact performance.
Linear impact performance is a function of the thickness or distance available to manage the impact. A common technique to improve helmet impact performance is to increase the standoff, or space between the shell and cranium. These helmets are called high standoff helmets. There is a limit to how big a helmet can be, however, and still be acceptable ergonomically, aesthetically, and from personal preferences. Many people prefer smaller helmets. Crushable foams and structures waste space. Crushable materials and structures generally do not crush enough to be effective. They typically have a fully crushed size that is too large, often as great as thirty percent of their pre-impact size even at the highest impact velocities called for in helmet standards. Helmets using such structures typically also leave extra space for fitment or comfort padding and positioning devices that have no functional role in active impact management.
Impact managing capabilities for crushable materials and structures is also a function of the breadth of the coverage area. The larger the coverage area, the greater the impact managing capability. Most crushable materials and structures have a coverage area of such extent that it inhibits heat transfer. Overheating is a common problem associated with these types of helmets.
Most protective headgear does not adequately manage oblique impacts, and oblique impacts may be one of the most common types of impact. By design, crushable materials and structures deform during an impact as the cranium “beds down” into the crushable material or structure in the process of managing the impact. This, in effect, fixes the head in place relative to the outer shell. Because of this, there is a logical and severe performance limit for these helmets to manage oblique impacts, which have both rotational and linear acceleration components.
A few methods have been proposed to try to mitigate this behavior. In one class, an attempt is made to provide more rotational freedom for the crushable impact liner to move relative to the hard outer shell. MIPS helmet technology adds a lower friction layer between the shell and crushable foam. In another method, described in U.S. Patent Application Publication No. 2012/0198604, an impact liner is divided into two concentric shapes with a flexible structure placed between them. A logical limit of both approaches is the asymmetrical shapes of heads and helmets that limit the amount of rotational movement between the hard shell and the crushable liner before there must be deformation (and therefore resistive force) of the crushable liner as it tries to rotate to an extent where the two shapes become increasingly mismatched. This shape mismatch is greater for lateral impacts because heads are more flat on the sides than on the top. Lateral impacts are arguably the most common of the oblique impacts. A further disadvantage of the method described in U.S. Patent Application Publication No. 2012/0198604 is that the standoff distance is increased significantly to accommodate the flexible standoffs between the layers. Many fitting means are also known that provide a secure fit but also further lock the head in pace relative to the outer shell, thereby, in most cases, limiting the helmet's ability to manage the rotational acceleration that is transmitted from the outer shell.
Superskin™ as provided by Lazer SA of Belgium seeks to lower the friction between the outer shell of a helmet and the impacting surface with the application of a lower friction gel like skin on the outside of the helmet. This can also be accomplished by making the outside of the helmet lower friction by other means such as using a harder shell, but using this approach will not mitigate all causes of rotational acceleration.
Shear thickening materials (e.g., d3o, Poron XRD) provide a rate sensitive response to different impact velocities. These materials may still suffer, however, from the other shortcomings of crushable foams and structures mentioned above, as well as having limited range. In helmet applications, they are mostly used to supplement, not replace, another crushable material or structure. A variation on a crushable structure is the vented air bladder of U.S. Pat. Nos. 7,895,681 and 3,872,511. These devices may provide improved rate sensitivity, but still have a minimal crush size, require a substantial size bladder and supporting bonnet, and are not as tunable as is desirable and possible with embodiments of the protective gear described herein.